3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE or wireless device, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
An important aspect of the communication industry today is the development of Internet of Things. That is, to connect devices, systems and services that goes beyond the traditional machine to machine, M2K communication. For example, a device like a light post is equipped with communication means so that it can be controlled remotely and so that it can communicate light failure. For it to be possible to equip such devices with communication means and still keep it at a reasonable cost, low cost User Equipments, UEs, are used. However, it has been pointed out that an important issue for low-cost UE operation is half-duplex Frequency Division Duplex, HD-FDD, mode. Low cost UEs should be kept as simple as possible and does therefore only have one oscillator which gives timing problems when switching between uplink and downlink in HD FDD.
Transmission and reception from a node, e.g. a terminal in a cellular system such as Long Term Evolution, LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). Frequency Division Duplex, FDD, as illustrated to the left in FIG. 1. Further objects, features, and advantages of the present disclosure will appear from the following detailed description, wherein some aspects of the disclosure will be described in more detail with reference to the accompanying drawings, in which:
FIG. 1 implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex, TDD, as illustrated to the right in FIG. 1. Further objects, features, and advantages of the present disclosure will appear from the following detailed description, wherein some aspects of the disclosure will be described in more detail with reference to the accompanying drawings, in which:
FIG. 1 implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
Typically, the structure of the transmitted signal in a communication system is organized in the form of a frame structure. For example, LTE uses ten equally-sized subframes of length 1 ms per radio frame as illustrated in FIG. 2.
In case of FDD operation (upper part of FIG. 2), there are two carrier frequencies, one for uplink transmission, fUL, and one for downlink transmission, fDL. At least with respect to the terminal in a cellular communication system, FDD can be either full duplex or half duplex. In the full duplex case, a terminal can transmit and receive simultaneously, while in half-duplex operation, the terminal cannot transmit and receive simultaneously (the base station is capable of simultaneous reception/transmission though, e.g. receiving from one terminal while simultaneously transmitting to another terminal). In LTE, a half-duplex terminal is monitoring/receiving in the downlink except when explicitly being instructed to transmit in a certain subframe.
In case of TDD operation (lower part of FIG. 2), there is only a single carrier frequency and uplink and downlink transmissions are always separated in time also on a cell basis. As the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An essential aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink nor uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions. For LTE, this guard time is provided by special subframes (subframe 1 and, in some cases, subframe 6), which are split into three parts: a downlink part, DwPTS, a guard period, GP, and an uplink part, UpPTS. The remaining subframes are either allocated to uplink or downlink transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations as shown in FIG. 3. Note that in the description below, DL subframe can mean either DL or the special subframe.
To avoid severe interference between downlink and uplink transmissions between different cells, neighbor cells should have the same downlink/uplink configuration. If this is not done, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell (and vice versa) as illustrated in FIG. 4. Hence, the downlink/uplink asymmetry can typically not vary between cells, but is signaled as part of the system information and remains fixed for a long period of time.
Currently in the LTE specification, the half-duplex FDD, HD-FDD, mode is not fully specified in terms of guard period.